Fast Radios Bursts (FRBs) show large dispersion measures (DMs), suggesting an extragalactic location. We analyze the DMs of the 11 known FRBs in detail and identify steps as integer multiples of half the lowest DM found, 187.5cm$^{-3}$ pc, so that DM
s occur in groups centered at 375, 562, 750, 937, 1125cm$^{-3}$ pc, with errors observed <5%. We estimate the likelhood of a coincidence as 5:10,000. We speculate that this could originate from a Galaxy population of FRBs, with Milky Way DM contribution as model deviations, and an underlying generator process that produces FRBs with DMs in discrete steps. However, we find that FRBs tend to arrive at close to the full integer second, like man-made perytons. If this holds, FRBs would also be man-made. This can be verified, or refuted, with new FRBs to be detected.
The distribution on the sky of unidentified sources at the highest energies where such a population is evident is investigated. For this purpose, sources without identification in the first Fermi-LAT catalog >10 GeV (1FHL) that are good candidates fo
r detection above the 50-100 GeV regime are selected. The distributions of these objects around the Galactic and super-galactic plane are explored. By using a Kolmogorov-Smirnov test it is examined if these sources are distributed homogeneously around these planes. Surprisingly, an indication for an inhomogeneous distribution is found for the case of the super-galactic plane where a homogeneous distribution can be excluded by a confidence level of 95%. On a 90% confidence level also a homogeneous distribution of sources around the Galactic plane can be excluded. For the hypothesis that this reflects the true distribution of sources rather than a statistical fluctuation, implications for the underlying source populations are discussed.
The nature of the gamma-ray source HESS J1507-622 that is located significantly off-set from the Galactic plane is not ascertained to date. Identifying the environment of an enigmatic object may help to constrain its nature. The path of the line of s
ight of HESS J1507-622 through the Galaxy is compared to the characteristic length scales of stellar populations of different ages. Furthermore, for this object, the energy density in particles is contrasted to the magnetic field energy density and constraints on the distance based on equipartition between these two components are calculated. The line of sight of HESS J1507-622 reaches a minimum distance to the Galactic center at around a galactocentric distance of 5.3 kpc at about 300 pc off the Galactic disc. This location coincides with the scale length and width of stars with an age of 1.2 Gyr which could in principle be an indication that HESS J1507-622 is connected to a stellar population of similar age. For such a case the source appears to be strongly particle dominated. In a leptonic scenario, if a magnetic field in the source of 1 $mu$G is assumed, equipartition between magnetic field and particles would be realized at a distance of >1 Mpc. This could indicate an extragalactic origin of this object. However, an extragalactic origin is challenged by the extension of the source. The environment of HESS J1507-622 still remains elusive. For the case where this source belongs to a new class of gamma-ray emitters, the distribution of related objects (if existing) may help to settle the respective environment and distance scale.
Nearby, Galactic gamma-ray bursts (GRBs) may affect the terrestrial biota if their radiation is beamed towards the Earth. Compact stellar binary mergers are possible central engines of short GRBs and their rate could be boosted in globular clusters.
Globular cluster typically follow well defined orbits around the galactic center. Therefore their position relative to the solar system can be calculated back in time. This fact is used to demonstrate that globular cluster - solar system encounters define possible points in time when a nearby GRB could have exploded. Additionally, potential terrestrial signatures in the geological record connected to such an event are discussed. Assuming rates of GRBs launched in globular cluster found from the redshift distribution of short burst and adopting the current globular cluster space-density around the solar system it is found that the expected minimal distance d_min for such a GRB in the last Gyr is in the range d_min ~ 1 - 3.5 kpc. From the average gamma-ray luminosity of a short GRB significant depletion of the terrestrial ozone-layer is expected if such an event explodes at a distance of ~1 kpc. In the last Gyr a few globular cluster passages are expected within a distance of d_min from the solar system and a GRB should have exploded during one of these passages. Globular cluster - solar system encounters and events of mass extinction in the history of life can be correlated to investigate the impact of a nearby GRB on the terrestrial biota. To explore such a correlation reliable globular cluster positions relative to the solar system have to be calculated for the time span of the fossil record of the last 600 Myr. The upcoming GAIA mission will be crucial to determine the possible time intervals of the occurrence of nearby GRBs launched in globular clusters.
Globular cluster are believed to boost the rate of compact binary mergers which may launch a certain type of cosmological gamma-ray bursts (GRBs). Therefore globular clusters appear to be potential sites to search for remnants of such GRBs. The very-
high-energy (VHE) gamma-ray source HESS J1747-248 recently discovered in the direction of the Galactic globular cluster Terzan 5 is investigated for being a GRB remnant. Signatures created by the ultra-relativistic outflow, the sub-relativistic ejecta and the ionizing radiation of a short GRB are estimated for an expected age of such a remnant of t > 10^4 years. The kinetic energy of a short GRB could roughly be adequate to power the VHE source in a hadronic scenario. The age of the proposed remnant estimated from its extension possibly agrees with the occurrence of such events in the Galaxy. Sub-relativistic merger ejecta could shock-heat the ambient medium. Further VHE observations can probe the presence of a break towards lower energies expected for particle acceleration in ultra-relativistic shocks. Deep X-ray observations would have the potential to examine the presence of thermal plasma heated by the sub-relativistic ejecta. The identification of a GRB remnant in our own Galaxy may also help to explore the effect of such a highly energetic event on the Earth.
Recently the H.E.S.S. collaboration announced the detection of an unidentified gamma-ray source with an off-set from the galactic plane of 3.5 degrees: HESS J1507-622. If the distance of the object is larger than about one kpc it would be physically
located outside the galactic disk. The density profile of the ISM perpendicular to the galactic plane, which acts as target material for hadronic gamma-ray production, drops quite fast with increasing distance. This fact places distance dependent constraints on the energetics and properties of off-plane gamma-ray sources like HESS J1507-622 if a hadronic origin of the gamma-ray emission is assumed. For the case of this source it is found that there seems to be no simple way to link this object to the remnant of a stellar explosions.
